The liquid crystal display device has a conventional, well-known problem of slow response. In the liquid crystal display device, the applied voltage to the liquid crystal layer is varied to change the alignment of liquid crystal molecules, which in turn results in a change in display pixel transmittance and hence a change in the display grayscale level. The slow response of the liquid crystal display device is caused by the liquid crystal molecules taking a prolonged period of time to completely change to a different alignment state after a change in the applied voltage to the liquid crystal layer.
Recent wide-screen and high-definition LCDs provide a very short drive period (write period) per pixel. Within the short write period, liquid crystal molecules cannot completely change their alignment in response to a change in the application voltage, and the LCD cannot achieve the desired display grayscale level.
There are approaches that are known to be capable of improving response speed. One example is overshoot driving for forceful grayscale level transition. The approach however entails angular response and other degradation in video quality and may not achieve sufficient speed. The approach also requires memory and other additional components.
To address these issues, for example, Japanese Unexamined Patent Publication (Tokukai) 2002-131721 (published May 9, 2002) discloses a method of improving the response speed by producing displays without using grayscale levels where the response speed slows down. The method is briefly explained.
The LCD's slow response problem does not occur uniformly across the whole range of grayscale levels. The response speed is extremely slow at some levels. For example, an LCD of vertical alignment (VA) in normally black mode exhibits extremely slow rising response speed when changing from a low grayscale level to a middle grayscale level, which can be a cause for after-images and other display problems.
Table 1 shows some results of measurement of the response speed of a VA module capable of producing 256 grayscale levels (0 to 255). The table shows changes of grayscale levels from one of nine levels (0, 32, 64, 96, 128, 160, 192, 224, and 255) to another of the nine levels.
TABLE 1“A”: Less than 1 frame (16.67 ms)“B”: 1 frame to less than 2 frames“C” 2 frames to less than 3 frames“F” 3 frames or longer
As can be seen from Table 1, the rising response speed is extremely slow, taking three or more frames to respond, when the grayscale level changes from 0 to middle level values (32, 64, 96, 128). Other entries showing an extremely slow rising response speed are mostly found where the grayscale level changes from a low value to a middle value.
Accordingly, the liquid crystal drive method of Tokukai 2002-131721 does not use those changes from low grayscale levels to middle grayscale levels where the rising response speed is slow. FIG. 4 identifies a typical range of liquid crystal driving voltage as A to B. In that convention, the range of liquid crystal driving voltage where the response speed is extremely slow is from A to C. The liquid crystal drive method of Tokukai 2002-131721 does not use the range A to C, but uses only the range C to B when driving liquid crystal. The voltage at A is not 0 volts because the response of the liquid crystal used with VA scheme is inherently slow in the absence of applied voltage.